[souffle]

It does sound as if there is something going on inside him and the weight loss is very worrying.
There have been a lot of cases of pet shop hamsters developing tumours lately and these can take them young. Could the vet feel anything internally? It usually is not possible to operate on things like bowel tumours as the bowel is very tiny in a hamster.
How are his teeth? Worth checking these are not overgrown and that they meet properly so he can eat well.
I would try offering some invalid foods to tempt him for now like babyfood, puried veggies, porridge, scrambled eggs etc and see if he will take these and build up a bit again.
You could have the vet scan him for anything inside but it is a lot to have him put under if he isn't so well.
It;s a difficult situation when there is nothing obvious the work on. Other things could be heart failure (hamsters are prone to faults in the heart) kidney failure or liver problems.
I hope he rallies round soon x